In recent years, as the output levels of internal combustion engines become higher, excessive rise in the temperature of the lubricating oil has come to be a major problem. To achieve a better cooling of the lubricating oil, an oil cooler is used in some cases. However, the use of an oil cooler means an increase in the manufacturing cost due to the increase in the number of component parts and the increased complexity of the manufacturing process. Furthermore, it leads to the increase in the weight of the engine which is not desirable when it is used as an automotive engine.
Additionally, particularly when die cast aluminum alloy is used as the material for the cylinder block, it is desired to achieve uniform cooling of the cylinder block to the end of preventing undue or local thermal expansions of various parts of the cylinder block. Excessive or local thermal expansion of a cylinder block causes an increase in the clearance between the cylinder bores and the pistons and this in turn could cause not only increased consumption of lubricating oil but also scuffing, seizing and so on.
Furthermore, to speed up the warm-up of the engine and the heater which makes use of the engine cooling water, it is desired to minimize the volume of the cooling water which circulates the engine cooling system.
These problems can be solved by providing a water jacket which extends substantially the whole length of the cylinders and reaches into the crank case while its width, the spacing between the outer surface of the cylinder block inner wall and inner surface of the outer wall of the cylinder block, is minimized. Thus, the lubricating oil in the crank case, as well as the wall of the cylinder bore in which combustion takes place, is effectively cooled without substantially increasing the volume of the water jacket. This is, however, extremely difficult to accomplish particularly when the cylinder block is made of die cast aluminum alloy since, during the casting process, the water jacket is defined by a core which forms a part of the casting die set and this core needs to be pulled out upon completion of the casting process. If the thickness of the core is small, it is difficult to assure a sufficient mechanical strength and rigidity to the core. Furthermore, as the core is pulled out against the frictional force it experiences from the mating surface of the water jacket, it could generate undue stress in the cast cylinder block and care must be taken to prevent generation of cracks in the part of the cylinder block which is adjacent to the water jacket.
U.S. Pat. No. 4,515,112 proposes an alternative approach to this problem. According to the invention of this U.S. Patent, a rib or ribs are provided in strategic places between the cylinder sleeve or the cylinder block inner wall and the cylinder block outer wall to connect them together for reinforcing the cylinder block inner wall defining cylinder bores to the end of eliminating uneven thermal deformation of the cylinder block inner wall. Japanese Utility Model Laid-Open Publication 57-43338 proposes the use of a filler member made of porous material in the water jacket in such a manner that the whole length of the cylinder may be evenly cooled. However, these approaches are not based on free circulation of cooling water and do not necessarily promote efficient cooling of the cylinder block.